Books:

1. R. E. Sherman, "Analytical Instrumentation", ISA Publication
2. R. S. Khandpur, "Handbook of Analytical Instruments", Tata McGraw-Hill Publication
3. A. K. Sawney, "Electrical and electronic Measurements and Instrumentation"

Useful links:

1. https://en.wikipedia.org/wiki/PH
2. https://www2.emersonprocess.com/siteadmincenter/PM%20Rosemount%20Analytical%20Documents/Liq_ADS_43-002.pdf